Two terminal programmable hot channel electron non-volatile memory

ABSTRACT

A programmable two terminal non-volatile device uses a floating gate that can be programmed by a hot electron injection induced by a potential between a source and drain. The floating gate layer can also function as a FET gate for other circuits in an integrated circuit containing an array of the devices. The invention can be used in environments such as data encryption, reference trimming, manufacturing ID, security ID, and many other applications.

RELATED APPLICATION DATA

The present application claims priority to and is a continuation of Ser.No. 12/869,469 now U.S. Pat. No. 7,920,426, which '469 applicationclaims priority to and is a continuation of Ser. No. 12/264,060 now U.S.Pat. No. 7,787,304, all of which are hereby incorporated by referenceherein. The '060 application in turn claims the benefit under 35 U.S.C.119(e) of the priority date of Provisional Application Ser. No.60/984,615 filed Nov. 1, 2007 which is hereby incorporated by reference.The '060 application is also related to the following applications, allof which are also hereby incorporated by reference herein:

INTEGRATED CIRCUIT EMBEDDED WITH NON-VOLATILE ONE-TIME-PROGRAMMABLE ANDMULTIPLE-TIME PROGRAMMABLE MEMORY (attorney docket no. JONK 2008-1) Ser.No. 12/264,029 now U.S. Pat. No. 7,782,668; and

METHOD OF OPERATING INTEGRATED CIRCUIT EMBEDDED WITH NON-VOLATILEONE-TIME-PROGRAMMABLE AND MULTIPLE-TIME PROGRAMMABLE MEMORY (attorneydocket no. JONK 2008-3) Ser. No. 12/264,076 to be issued as U.S. Pat.No. 7,787,309.

FIELD OF THE INVENTION

The present invention relates to making non-volatile memories which canbe programmed one time, or multiple times in some instances. Theinvention has particular applicability to applications where is itdesirable to customize electronic circuits.

BACKGROUND

One time programmable (OTP) and multi-time programmable (MTP) memorieshave been recently introduced for beneficial use in a number ofapplications where customization is required for both digital and analogdesigns. These applications include data encryption, reference trimming,manufacturing ID, security ID, and many other applications.Incorporating OTP and MTP memories nonetheless typically comes at theexpense of some additional processing steps.

An NMOS OTP implementation is disclosed by U.S. Pat. No. 6,920,067,incorporated by reference herein. The device in this reference isprogrammed with channel hot-hole-injection. The disclosure teaches thatthe device is programmed into conducting state, after the channel hothole injection. However, it is unclear whether the device actually worksin the way the inventors claim. That is, it is not apparent that thechannel current will be initiated to induce hot-hole-injection since thestate of the floating gate is unknown and there is no available means tocouple a voltage unto the floating gate. An NMOS device will conduct achannel current to initiate the hot hole injection only when thefloating gate potential is sufficient to turn on the device, or when thethreshold voltage is always low initially to allow channel currentconduction. The only way to ensure either scenario is to introduce anadditional process step to modify the turn on characteristics of theNMOS. Now assuming the channel is conducting initially and hot holes areinjected, the holes injected on the floating gate will make the devicemore conductive. So the device basically goes from a conductive state(in order to initiate channel current for hot hole injection) to ahighly conductive state. This is not a very optimal behavior for amemory device.

Another prior art device described in U.S. publication no. 2008/0186772(incorporated by reference herein) shows a slightly different approachto the problem of providing a programming voltage to a floating gateembodiment of an OTP device. In this design, shown in FIG. 4, the drainborder length L1 is increased relative to the source side length L1 toincrease a coupling ratio to the eraseable floating gate 416. Byincreasing the coupling ratio, the amount of channel current isincreased; therefore the charge injection into the floating gate willalso increase. The drawbacks of this cell, however, include the factthat the cell and channel 412 must be asymmetric, and the coupling isonly controlled using the length dimension of the active regions.Because of these limitations, it also does not appear to be extendableto a multi-level architecture. Moreover, it apparently is onlyimplemented as a p-channel device.

Accordingly there is clearly a long-felt need for a floating gate typeprogrammable memory which is capable of addressing these deficiencies inthe prior art.

SUMMARY OF THE INVENTION

An object of the present invention, therefore, is to overcome theaforementioned limitations of the prior art.

A first aspect of the invention concerns a programmable non-volatiledevice situated on a substrate comprising: a floating gate; wherein thefloating gate is comprised of a material that is also used as a gate fora transistor device also situated on the substrate and associated with alogic gate and/or a volatile memory; a source region; and a drainregion; and an n-type channel coupling the source region and drainregion; wherein the drain region overlaps a sufficient portion of thegate such that a programming voltage for the device applied to the draincan be imparted to the floating gate through capacitive coupling.

Preferably the programming voltage is greater than 5 volts. In someinstances the floating gate can be erased to allow for reprogramming.The floating gate is eraseable by an erase voltage applied to the sourceregion.

The state of the floating gate can be determined by a read signalapplied to the drain which is preferably less than about 1 volt.

The inventive device can be part of a programmable array embedded withseparate logic circuits and/or memory circuits in an integrated circuit.The data stored in the memory can be used as a part of (or by) a dataencryption circuit; a reference trimming circuit; a manufacturing ID; asecurity ID or other similar applications.

In some embodiments the capacitive coupling can be configured to placein a first trench situated in the substrate. A separate set of secondtrenches in the substrate can be used as embedded DRAM.

The programmable device can be coupled to a second programmable devicein a paired latch arrangement such a datum and its compliment are storedin the paired latch.

In some embodiments the floating gate is being comprised of a materialthat includes impurities acting as charge storage sites and is also usedas an insulating layer for other non-programmable devices situated onthe substrate, such as an oxide. In other applications the floating gateis comprised of a material that is also shared by an interconnect and/oranother gate for a transistor device also situated on the substrate andassociated with a logic gate and/or a volatile memory.

Another aspect of the invention concerns a one-time (OTP) or multi-time(MTP) programmable memory device incorporated on a silicon substratewith one or more other additional logic and/or non-OTP memory devices,characterized in that the OTP memory device has an n-type channel; anyand all regions and structures of the OTP memory device are derivedsolely from corresponding regions and structures used as components ofthe additional logic and/or non-MTP/OTP memory devices.

Another aspect of the invention concerns a method of forming the aboveNV OTP/MPT device situated on a substrate comprising the followingsteps: forming a gate for non-volatile programmable memory device from afirst layer; the first layer being shared by the non-volatileprogrammable memory device and at least one other device also situatedon the substrate and associated with a logic gate and/or a volatilememory; forming a drain region; and capacitively coupling the gate withthe drain region by overlapping a portion of the gate with the drainregion.

As noted above, the first layer is preferably polysilicon, or aninsulating layer which has impurities introduced during a source ordrain implant step. The device is formed with n-type channel.

Preferably the non-volatile programmable memory device is embedded in acomputing circuit and formed entirely by CMOS processing and masks usedto form other logic and/or memory n-channel devices in the processingcircuit.

The non-volatile memory can be programmed during manufacture if desiredto store one or more identification codes for a wafer, and/or can beassociated with one of the following: a data encryption circuit; areference trimming circuit; a manufacturing ID; and/or a security ID.

In other embodiments all regions and structures of the OTP memory deviceare formed in common with corresponding regions and structures used ascomponents of the additional logic and/or non-OTP memory devices.

A further aspect concerns a method of operating a non-volatileprogrammable (NVP) device situated on a substrate comprising: providinga floating gate, which floating gate is comprised of a layer andmaterial that is shared by gates of at least some other non-NVP deviceson the substrate; programming the NVP device to a first state withchannel hot electrons that alter a voltage threshold of a floating gate;reading the first state in the OTP device using a bias current to detectthe voltage threshold; and erasing the NVP device with band-bandtunneling hot hole injection.

In preferred embodiments the floating gate is comprised of a materialthat is also used as a gate for a transistor device also situated on thesubstrate and associated with a logic gate and/or a volatile memory. Asubstantial portion of the programming voltage applied to the drain isalso imparted to the floating gate through the capacitive coupling. Inpreferred embodiments the threshold of the floating gate is set by acurrent of channel hot electrons to store data in the OTP device.

It will be understood from the Detailed Description that the inventionscan be implemented in a multitude of different embodiments. Furthermore,it will be readily appreciated by skilled artisans that such differentembodiments will likely include only one or more of the aforementionedobjects of the present inventions. Thus, the absence of one or more ofsuch characteristics in any particular embodiment should not beconstrued as limiting the scope of the present inventions. Whiledescribed in the context of a non-volatile memory array, it will beapparent to those skilled in the art that the present teachings could beused in any number of applications.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top down view of a preferred embodiment of a non-volatilememory cell of the present invention;

FIG. 2 is a side cross section view of the preferred non-volatile memorycell;

FIG. 3 is an electrical diagram illustrating the electrical relationshipof the structures of the preferred non-volatile memory cell;

FIG. 4 depicts a prior art non-volatile memory cell which uses afloating gate for an OTP application;

FIG. 5 is an electrical diagram showing a preferred embodiment of alatch circuit constructed with the NV memory cells of the presentinvention.

DETAILED DESCRIPTION

The present disclosure concerns a new type of non-volatile memory devicestructure (preferably single poly) that can be operated either as an OTP(one time programmable) or as an MTP (multiple time programmable) memorycell. The preferred device structure is fully compatible with advancedCMOS logic process, and would require, at the worst case, very minimaladditional steps to implement.

A unique aspect of the present device is that the floating gate of thememory cell structure is electrically coupled strongly through one ofthe S/D junctions of the transistor, whereas traditional single polynonvolatile memory cells require either an additional interconnect layerto couple to the floating gate, or the floating gate has virtually noneor minimal electrical coupling to any of the existing electricalsignals. Moreover, unlike the 2008/0186772 reference, the coupling ratiocan be more specific and precise. That is, by exactly controlling thecoupling ratio (through areal means) the amount of charge, and thus thefinal programmed Vt, are directly proportional to the product of thecoupling ratio and the drain voltage. It can be more preciselycontrolled such that the coupling ratio is dictated or designed by thedesired programming threshold level (V_(t)) of the memory cell. Thisallows for a design that evolves easily into a multi-level version of anOTP since different coupling ratios yield different programmed V_(t).

FIG. 1 illustrates the top view of the layout of a preferred structureused in the present invention. FIG. 2 illustrates a representativecross-sectional view of the device structure. It will be understood thatthese drawings are not intended to be set out to scale, and some aspectsof the device have been omitted for clarity.

The device includes a typical NMOS transistor 100 which is modified sothat the gate (poly in a preferred embodiment) 110 of the device is notelectrically connected to a voltage source. A drain 120 of the device isbent around and is preferably joined by an N-type well 130 thattypically already exists in a conventional advanced CMOS process. As analternative, the N-Well 130 can be replaced with an n-type diffusionlayer introduced so as to be beneath the poly floating gate. Aconventional source region 125 is also utilized.

The floating gate poly 110 is extended beyond a typical transistorchannel region 135 and includes an overlap region 140 which overlaps anactive region extending from the drain junction. The active regionportion 141 that is surrounded by the N-Well region serves as aneffective capacitive coupling to the floating gate. Thus any voltageapplied to the drain junction will be effectively coupled onto thefloating gate.

As seen in the electrical diagram of FIG. 3, if the coupling ratio ofthe drain to the floating gate is sufficiently high—which is determinedby the ratio of the area of the gate channel region and the area of thePoly extension overlapping the drain extension region—the floating gatecan effectively acquire and have a high percentage of the value of thedrain voltage.

A key advantage of the preferred embodiment, as seen in FIGS. 1 and 2,is that it is formed from same layers conventionally used to make activen-channel devices in a CMOS process. The only difference is that thepoly (or metal as the case may be) gate layer is not interconnected withsuch other formed active devices or coupled to a gate signal. The otherimplants for the source/drain are also part of a CMOS conventionalprocess. Thus, in most applications the invention can be integratedwithout any additional processing costs, because the only alteration isto an existing mask for each relevant layer of the wafer beingprocessed.

One other optional variation of this device structure is to make thedrain-to-gate coupling capacitor area on the sidewall of a trench. Thiswill greatly reduce the area of the drain-to-gate coupling capacitor.This reduction in cell area may come at the expense of significantlyincrease the manufacturing process complexity. However, again, inapplications where the invention is integrated with certain types ofDRAM architectures (especially embedded types), it is possible toincorporate the conventional processing steps for such memories to avoidadditional processing costs. Other techniques for coupling a voltage tothe floating gate and achieving a desired coupling ratio will beapparent to those skilled in the art.

While the floating gate is shown as a single polysilicon layer, it willbe appreciated by skilled artisans that other materials could be used aswell. In some applications for example it may be possible to exploit theformation of other structures/devices which while part of other mainunderlying logic/memory structures, can be exploited for purposes ofmaking a floating gate of some kind. In this respect it should be notedthat floating gates can typically be formed of a number of differentmaterials, including through techniques in which impurities areimplanted/diffused into a dielectric/insulating layer.

Moreover while the preferred embodiment depicts the NVM cell as part ofa conventional lateral-planar FET structure on a substrate, it will beapparent to those skilled in the art that other geometries/architecturescan be used, including non-planar structures. Thus the invention couldbe used in SOI substrates, in thin film structures, at other levels ofthe device than the substrate, in multi-gate (FINFET type) orientations,and in vertical/non-planar configurations. In such latter instances thefloating gate would be embedded and oriented vertically with respect tothe substrate.

The preferred operation of device 100 will be described. Thenon-volatile device structure preferably has the physical features of aconventional I/O transistor implemented in an advanced CMOS logicprocess. At present, such I/O transistor is nominally operated at 3.3Vbut it will be understood that this value will change with successivegenerations of manufacturing.

This type of I/O transistor typically has a threshold voltage of 0.5V to0.7V, with a typical electrical gate oxide thickness of 70 A. With adrain coupling to floating gate ratio of 0.90, and a read drain voltageof 1.0V applied to the device, the floating gate will effectively becoupled with a voltage of about 0.90V. This is sufficient to turn on theun-programmed NMOS device 100, and a channel current can be detected bytypical means of sense circuitry to identify the state of the device. Itwill be understood to those skilled in the art that the particularcoupling ratio, read voltage, etc., will vary from application toapplication and can be configured based on desired device operatingcharacteristics.

The device is originally in a unprogrammed state, which in the preferredembodiment is characterized by a low resistance coupling between thesource and drain through channel region 135. This means that the channelregion 135 can be substantially uniform and current flow is reliable.While the preferred embodiment is shown in the form of a symmetriccell/channel, it will be understood that the invention could be used innon-symmetric forms such as shown in the aforementioned 20080186722publication.

To program the device into a programmed state, the device must be shutoff by reducing carriers in the channel region, and increasing thethreshold voltage. To do this a drain voltage of 6.0V can be applied andthis will effectively couple a voltage of about 5.4V to the floatinggate. This bias condition will placed the device into a channel hotelectron injection regime. The electrons injected into the floating gateeffectively increase the threshold voltage of the device. When asubsequent read voltage of 1.0V is applied again on the drain, thedevice does not conduct current due to its high threshold voltage, andthis second state of the device is thus determined. As with the readcharacteristics, it will be understood to those skilled in the art thatthe particular coupling ratio, program voltage, etc., will vary fromapplication to application and can be configured based on desired deviceoperating characteristics.

The prior art referred to above is primarily a one time programmabledevice, since there is no disclosed mechanism for removing the charge onthe floating gate. In contrast, some embodiments of the presentinvention can be made to be capable of multiple-time-programming. To dothis, an erase operation can be introduced to remove or neutralize theelectrons that have been injected into the floating gate. The mechanismfor removing or neutralizing electrons is preferably through band-bandtunneling hot hole injection from the other non-coupling junction 125 ofthe device. The preferred bias condition would be as followed: thenon-coupling junction (source junction) is biased with 6V to cause thejunction to initiate band-band tunneling current. The band-bandtunneling current causes hot holes to be injected into the floating gateand neutralize the electrons that are stored on the floating gate. Thusit is (re)programmed from a non-conducting, or even a low conductingstate, into a conducting state. The device is then able to conductchannel current when a subsequent read voltage is applied to thecoupling junction during the read operation. It will be understood thatprogramming from a low conducting state to a conducting state may have alimited operating sense window.

As an additional optional operation, to facilitate erase operation andenhance band-band tunneling current, the coupling junction can besupplied with a negative voltage so that the floating gate is made morenegative to cause higher band-band tunneling current across the sourcejunction.

Thus the operating characteristics are preferably as follows:

OPERATION Drain Source Substrate Program 6.0 V 0 V 0 V Read 1.0 V 0 V 0V Erase Float or -Vcc 6.0 V   0 V

In some embodiments, additional protection can be implemented to ensurethe OTP and MTP device have sufficient immunity against the loss ofcharge stored on the floating gate. To do this, the device can beconfigured into a paired latch 500—as shown in FIG. 5—where the data andits complement are stored into the latch, thus effectively doubling themargin in the stored data. As seen therein, a top device 510 couples anode 530 to a first voltage reference (Vcc) while a second bottom device520 couples the node to a second voltage reference (Vss). By placingcharge on the top device floating gate, the top device 510 is programmedinto a non-conductive state, thus ensuring that node 530 is pulled downby bottom device 520 to Vss, representing a first logical data value(0). Similarly, by placing charge on the bottom device floating gate,the bottom device 520 is programmed into a non-conductive state, thusensuring that node 530 is pulled up by top device 510 to Vcc,representing a second logical data value (1).

Another useful advantage of the present preferred embodiment is that itis implemented with an NMOS device structure, whereas most traditionalsingle-poly OTPs are commonly implemented with a PMOS device structure.This means that the device can be formed at the same time as othern-channel devices on a wafer. Another advantage of an NMOS devicestructure in this invention is that it behaves similar to an EPROMdevice, i.e., the device is programmed into a non-conducting state froma conducting state. In contrast, the prior art 20080186722 typedevice—and other commonly used PMOS OTP devices—are programmed from anon-conducting state into a conducting state.

This aspect of the invention thus can eliminate the need of anadditional masking step that is commonly associated with a PMOS OTPdevice in order to make sure that PMOS device is in a non-conductingstate coming out of the manufacturing fab.

In addition, since an NMOS device's programming mechanism with channelhot electrons injection is self-limiting, unlike that case of a PMOSwith channel hot electron programming, the amount of energy consumptionduring programming is self-limited for this invention.

As seen in the present description therefore, the particularconfiguration of the floating gate is not critical. All that is requiredis that it be structurally and electrically configured to controlchannel conduction and also be capacitively coupled to an electricalsource of charge carriers. The particular geometry can be varied inaccordance with any desired layout or mask. In some instances it may bedesirable to implement the floating gate as a multi-level structure forexample. Moreover, since capacitive coupling is a function of thematerials used, the invention allows for significant flexibility as thecomposition of the floating gate can also be varied as desired toaccommodate and be integrated into a particular process. An array ofcells constructed in accordance with the present teachings could includedifferent shapes and sizes of floating gates so that cells havingthreshold cells could be created.

The above descriptions are intended as merely illustrative embodimentsof the proposed inventions. It is understood that the protectionafforded the present invention also comprehends and extends toembodiments different from those above, but which fall within the scopeof the present claims.

1. A two terminal programmable non-volatile device situated on asubstrate comprising: a first diffusion region coupled to a firstterminal; and a second diffusion region coupled to a second terminal;and a channel coupling said first diffusion region and second diffusionregion; a floating gate comprised of polysilicon, which floating gate issituated over at least a portion of said channel; wherein the device isadapted such that a channel hot electron current can be induced in saidchannel by a voltage potential imposed across said first terminal andsaid second terminal by a single program voltage, and which voltagepotential is sufficient to cause injection into said floating gate andprogram the device.
 2. The programmable device of claim 1 wherein saidsingle program voltage can be applied across said first terminal andsaid second terminal.
 3. The programmable device of claim 1 wherein asubstrate under said channel can be biased to enhance hot electrongeneration.
 4. The programmable device of claim 1 wherein charge can beimparted to said floating gate by single program voltage through arealcapacitive coupling between said floating gate and said first diffusionregion.
 5. The programmable device of claim 1 wherein the firstdiffusion region overlaps a sufficient areal portion of said floatinggate to permit a threshold voltage of the device to be controlled by acoupling ratio determined by said areal portion and said single programvoltage applied to such second diffusion region.
 6. The programmabledevice of claim 1 wherein said device can be re-programmed.
 7. Theprogrammable device of claim 1 wherein a state of said floating gate canbe determined by a read signal applied to said second diffusion region.8. The programmable device of claim 7, wherein said read signal is lessthan about 1 volt.
 9. The programmable device of claim 1 wherein saiddevice is part of a programmable array embedded with separate logiccircuits and/or memory circuits in an integrated circuit.
 10. Theprogrammable device of claim 1 wherein said device is part of aprogrammable array embedded with separate analog circuits within anintegrated circuit.
 11. The programmable device of claim 1 wherein saidfloating gate is comprised of a polysilicon layer that also used forlogic gates within the same integrated circuit.
 12. The programmabledevice of claim 1 wherein said device is associated with one of thefollowing: a data encryption circuit; a reference trimming circuit; amanufacturing ID; and/or a security ID.
 13. The programmable device ofclaim 1, wherein a threshold voltage of said device can be set tomultiple discrete levels.
 14. The programmable device of claim 1,wherein the device is part of a programmable memory array.
 15. A twoterminal programmable non-volatile device situated on a substratecomprising: a first n-type diffusion region coupled to a first terminal;and a second n-type diffusion region coupled to a second terminal; andan n-type channel coupling said first diffusion region and seconddiffusion region; a floating gate comprised of polysilicon, whichfloating gate is situated over at least a portion of said channel and isadapted to store electrical charge corresponding to a logic state of thedevice; further wherein the device is adapted such that a channel hotelectron current can be induced in said channel sufficient to causeinjection into said floating gate using a single programming voltageapplied only to at most one of said first and second terminals.
 16. Aone-time programmable (OTP) two terminal memory device incorporated on asilicon substrate with one or more other additional logic and/or non-OTPmemory devices, characterized in that: a. the OTP memory device has afirst diffusion region and a second diffusion region; b. the OTP memorydevice has a floating gate overlying at least in part a channel regionextending between said first diffusion region and said second diffusionregion; wherein said channel is adapted to have a low resistance whenthe device is in an unprogrammed state, and a high resistance when thedevice is in a programmed state; and c. any and all regions andstructures of said OTP memory device are derived solely fromcorresponding regions and structures used as components of theadditional logic and/or non-OTP memory devices, including said n-typediffusion region which is used by said devices; d. the OTP memory devicecan be set to said programmed state by hot channel electron injectioninduced by a potential between said first diffusion region and saidsecond diffusion region caused by a single programming voltage coupledto the device.
 17. The OTP memory device of claim 16 wherein said singleprogramming voltage results in a programming voltage being coupledthrough an areal portion between said first diffusion region and saidfloating gate.
 18. The OTP memory device of claim 16 wherein a couplingratio in said areal portion is at least 90%.
 19. The OTP memory deviceof claim 16 wherein said floating gate can be erased.
 20. The OTP memorydevice of claim 16 wherein said device can be re-programmed.
 21. The OTPmemory device of claim 16 wherein said floating gate is eraseable by anerase voltage applied to said source region.
 22. The OTP memory deviceof claim 16 wherein a state of said floating gate can be determined by aread signal applied to said first diffusion region.
 23. The OTP memorydevice of claim 16 wherein said device is associated with one of thefollowing: a data encryption circuit; a reference trimming circuit; amanufacturing ID; and/or a security ID.
 24. The OTP memory device ofclaim 16, wherein a threshold voltage of said device can be set tomultiple discrete levels.